1. Field of the Invention
This invention generally relates to a text to speech conversion system for converting letter image or phonetic symbols etc into voice, and particularly to a system capable of distinguishing geographical names based upon the present position.
2. Description of the Related Art
In a conventional Car Navigation System (CNS), the system receives coordinates of the present position of a mobile unit sent by Global Positioning System (GPS) and then indicates the position in a map on a display monitor such as a CRT etc. The above described CNS provides guidance such as various road-related information with voice for safety driving. Furthermore, the Vehicle Information and Communication System (VICS) such as disclosed in NIKKEI ELECTRONICS (Vol. 622, 1996. 5.20, P91 ? P106) is generally beginning to put into use. The VICS, which does not have a database including phonetic symbols corresponding to messages of the guidance, delivers text to speech conversion.
Japanese Laid Open Publication 63-259,412 also discloses another example of the above systems. Japanese Open Laid 63-259,412 shows both a present position of a mobile unit on a route to a destination. The system also selects a proper route for the destination, provides the information on the selected route and then guides by voice according to the present position on the selected route. The above information guidance only outputs a limited number of fixed phrases of messages in the system. In contrast, the system needs an enormous amount of voice data in order to provide various route information by recorded voice. In particular, the system has to inform drivers of a large number of geographical names. Since, the system can not hold an enormous amount of voice data representing geographical names, the above-described systems are not capable of adopting the VICS. For this reason, some additional text to speech conversion systems are proposed to solve the geographical name problem.
Japanese Laid Open Publication 08-76,796 discloses a system which separates a sentence with variable-length messages and fixed-length messages and then acoustically synthesizes voice data by transforming the variable-length messages based upon voice synthesis rules and voice data corresponding to the fixed-length messages. However, the above system is unable to distinguish nouns that have plural pronunciations with the same notation. This problem is especially common among Japanese characters used for geographical names. Other systems are proposed to solve the above problem in Japanese Laid Open Publications 06-289,890 and Japanese Laid Open 7-36,906. Japanese Laid Open 06-289,890 discloses an apparatus which distinguishes certain Japanese characters that have plural pronunciations with the same notation. The system has an idiomatic expression dictionary. When the system analyzes sentences containing nouns that have plural pronunciations with the same notation, the system distinguishes the nouns based upon an idiomatic relation with respect to adjacent words. On the other hands, the Japanese Open Laid 7-36,906 discloses a distinction system by changing an order of priority of Japanese characters in a word dictionary that have plural pronunciations for the above described sentence analysis. In other words, the system intends to distinguish plural pronunciations by changing the order of priority of the Japanese characters stored in the word dictionary.
Nevertheless, the above systems are unable to correctly distinguish nouns that have plural pronunciations with the same notation. Furthermore, the Japanese Laid Open Publication 7-36,906 does not clearly disclose how distinctions are correctly made based upon the modified order of priority of the words that have plural pronunciations with the same notation.
In the above described systems, the systems have a dictionary which stores the words that have plural pronunciations with the same notation and distinguishes the words by a word retrieval function of the word dictionary in the morpheme analysis. In other words, the above systems distinguish the words that have plural pronunciations with the same notation based upon the morpheme analysis during the word retrieval from the word dictionary so as to read the information that is inputted by the VICS etc. As already mentioned, when the words are geographical names, many of them have plural pronunciations with the same notation in the Japanese language. In contrast, geographical names have the same notation and the same pronunciation even though locations corresponding to the word differ in the English language. For example, there are the geographical names such as Texas, Connecticut, Arlington in the United States of America. Inventors of the present invention identified that nouns representing geographical names that have plural pronunciations with the same notation or the same pronunciation but different locations be precisely distinguished based upon the context. For example, of Tokyo Metropolis of Japan (pronunciation is "MITA") and of Hyogo Prefecture of Japan pronunciation is "SANDA"). The word dictionary has to uniquely store such that notation in the above systems. Accordingly, the system needs to select one of the plural pronunciations with the same notation, and stores only a selected notation into the word dictionary.
Furthermore, inventors of the present invention also identified that nouns representing geographical names having same notation and same pronunciation desired to correctly distinguish these geographical names based upon the context. Mispronunciation of geographical names becomes a fatal disadvantage in the car navigation system.